CN111712982A

CN111712982A - Fault handling

Info

Publication number: CN111712982A
Application number: CN201980014880.9A
Authority: CN
Inventors: C·M·索纳蒂; R·A·穆赫德卡尔
Original assignee: General Electric Technology GmbH
Current assignee: General Electric Technology GmbH
Priority date: 2018-02-23
Filing date: 2019-02-12
Publication date: 2020-09-25
Anticipated expiration: 2039-02-12
Also published as: US20200395757A1; CN111712982B; WO2019162148A1; EP3531523B1; EP3531523A1; US11770005B2

Abstract

The present application relates to methods and apparatus for handling faults associated with Voltage Source Converters (VSCs) for exchanging electrical power between AC systems (101, 102) and DC systems (106-1, 106-2). The VSC (104) is connected to the AC system via an interface device comprising a transformer (107) having a set of primary windings (202) for coupling to a plurality of AC phases (A, B, C) of the AC system. In an embodiment of the disclosure, the primary winding set has a neutral point (N) and the interface device comprises a fault module (301), the fault module (301) having an energy storage element (302), the energy storage element (302) being connected in parallel with a resistive element (303) between the neutral point of the primary winding set and a reference voltage, such as ground. The fault module does not interfere with normal operation, but in the event of a phase ground fault on the secondary side of the transformer, it is possible to induce earlier zero crossings in the AC phase than would otherwise result, thus allowing the AC circuit breaker (108) to open with a shortened arcing time.

Description

Fault handling

Technical Field

The present disclosure relates to a method and apparatus for fault handling, and in particular to a method and apparatus for handling faults in a connection between an AC system and a DC system, and in particular to handling faults associated with a voltage source converter.

Background

Generally, distribution and transmission of electric power has been achieved mainly by using Alternating Current (AC) distribution schemes, in particular, high voltage AC distribution systems. Direct Current (DC) systems, and in particular HVDC (high voltage direct current), are increasingly being proposed for some electrical power transmission applications. There may be many benefits of using HVDC electrical power.

In order to use HVDC electric power transmission, it is typically necessary to convert AC electric power to DC and back again. Thus, the DC system may be coupled to one or more AC systems, with at least one converter station at each interface between the AC system and the DC system.

Historically, HVDC systems have been implemented using line commutated converters in converter stations, using elements such as thyristors which can be controllably turned off but which remain conductive as long as they are correctly biased by the line voltage. However, developments in the field of power electronics have led to increased use of Voltage Source Converters (VSCs) for AC-DC conversion and DC-AC conversion. VSCs use switching elements, typically Insulated Gate Bipolar Transistors (IGBTs) that can be controllably turned on and off, connected with respective anti-parallel diodes.

Typically, a VSC will have a phase branch for each electrical phase of the AC system. Typically, the AC system will be a three-phase electrical system. The AC node of each converter arm may be connected to a respective AC phase of the AC system via a transformer having a set of primary windings electrically connected to the AC system and a set of secondary windings electrically connected to a phase limb of the VSC.

Each phase limb may be coupled to the DC system at respective high-side and low-side DC terminals (e.g., between a high-side DC bus bar and a low-side DC bus bar). The phase branch will typically comprise a plurality of converter arms, wherein each converter arm extends between an AC node and a defined nominal DC voltage. In a symmetrical unipolar arrangement, the phase leg may have two converter arms, a high side converter arm extending between the AC node and the high side DC terminal and a low side converter arm extending between the AC node and the low side DC terminal. In a bipolar arrangement, two converter arms may be arranged between the high-side DC terminal and a reference potential (e.g., ground), with the AC node at the midpoint of the two arms. The other two converter arms may be located between the reference potential and the low side DC terminal. Each converter arm includes a switching device called a valve. In some VSC designs, such as Modular Multilevel Converters (MMC) or Alternating Arm Converters (AAC), the valve may be formed at least in part by a series connection of a plurality of cells, each cell having an energy storage element, such as a capacitor that can be selectively connected in series or bypassed (bypass) between the terminals of the cell. The valve can be used for voltage waveform shaping (wave-shaping) and can allow switching between AC and DC with relatively low distortion.

In some VSC designs, circuit breakers in or coupled to the AC system (i.e. circuit breakers connected on the primary winding side of the transformer) act as primary protection in case a fault is detected in the AC system or VSC. An AC circuit breaker operable in a high voltage system typically comprises a mechanical disconnector having two electrical contacts (contacts) movable with respect to each other. In normal operation, the switch is closed and the two electrical contacts are in physical contact with each other to provide an electrically conductive path with low on-resistance. In the event of a fault, the switch is opened and one contact is moved relative to the other to break the physical contact and prevent conduction. However, it takes a period of time for the switch to open, and due to the high voltages involved, when the two electrical contacts have been physically severed but are still close to each other, the electric field between the contacts may be very high and above the breakdown voltage of the medium (e.g., air) between the contacts. Arcing between the contacts (arcing) may occur if the circuit breaker is opened while current flows through the circuit breaker, and once arcing has been established, the arc may be maintained as long as current continues to flow to the circuit breaker.

In AC systems, the current naturally crosses zero twice in each cycle. When the AC current drops to zero, the arcing is terminated. However, under certain fault scenarios that may occur within an HVDC converter station, the fault current through the circuit breaker may not have zero crossings at least for several AC cycles. This means that the currents in those phases may have delayed zero crossings, or may not have zero crossings at all, meaning that the AC circuit breaker may not operate efficiently and may experience sustained arcing durations.

Both patent publications US2013/0063989 and US2014/0268942 address this problem and describe interface arrangements for coupling between AC and DC systems. Both of these documents disclose the use of additional switching devices, i.e. elements that are actively (activery) controlled in case of a fault, to provide a short circuit to ground on the secondary side of the transformer as described in US2013/0063989 or via an additional set of windings of the transformer as described in US 2014/0268942. The additional switching devices are disadvantageous because they increase the cost and footprint of the VSC and also result in additional variables where the time taken to clear the fault depends on the speed of the additional switching devices. WO2016066196 also solves this problem with a control-based solution without the need for additional switching devices. However, the proposed solution relies on delaying the breaker opening time, which thus disadvantageously increases the duration of the fault. Therefore, an alternative solution would be advantageous.

Disclosure of Invention

Embodiments of the present disclosure relate to improvements in fault handling in response to faults.

Thus, in one aspect, there is provided an interface device for interfacing between an AC system and a DC system, the device comprising:

a transformer having a winding set for coupling to a plurality of AC phases of the AC system, the winding set having a neutral point; and

a fault module comprising an energy storage element connected in parallel with a resistive element between a first module node and a second module node;

the first module node is electrically connected to the neutral point of the winding set and the second module node is connected to a reference node which in use is held at a reference voltage.

During normal operation, the voltage at the neutral point of the primary winding set remains balanced and the fault module has no substantial effect on normal operation. In case of a fault of the type discussed above, the AC circuit breaker in series with the AC phase can be commanded to open. As noted above, one AC phase will be expected to have a zero crossing of the current and thus the associated circuit breaker will open successfully in a relatively short time, but in at least one of the other phases there may be a substantial DC component of the fault current. In embodiments of the present disclosure, however, there will be an imbalance at the neutral point once the AC circuit breaker for one phase has been opened. Thus, current can flow to or from the energy storage element. This current flow induced (induce) by the fault module affects the fault current distribution in the other AC phases, so that the remaining AC phases will be expected to experience a zero crossing of the current earlier than would otherwise result. Thus, the interface device of an embodiment allows to command all AC circuit breakers to open in case of a fault, thus providing a simple control method, but providing a more rapid opening of all AC circuit breakers with a reduced arcing time.

The fault module may further include a surge arrester electrically connected in parallel with the energy storage element and the resistive element between the first module node and the second module node. The energy storage element may comprise at least one capacitor.

The interface device may further include: a respective AC circuit breaker for each of the AC phases of the AC system; and a fault controller for operating the AC circuit breaker in the event of a fault.

The winding of the transformer to which the fault module is coupled may be a primary winding set, in other words, the fault module is located on the primary winding side of the transformer.

The interface device may further comprise a set of secondary windings of the transformer for coupling to a plurality of phase branches of a voltage source converter.

In the event of any of the phase-to-ground faults occurring in any of the following, the fault controller may be configured to command the AC circuit breaker to open: the set of secondary windings; a phase leg of the voltage source converter or a connection between the set of secondary windings and a phase leg of the voltage source converter. In case of such a fault, the fault controller may also be configured to send a command to block the voltage source converter.

The energy storage element and the resistive element of the fault module may be configured such that in case one of the AC circuit breakers is open, the energy storage element generates (source) or sinks (sink) a current sufficient to modify the current distribution of the various phases in order to cause a zero crossing of the current in at least one of the other phases, such that each AC circuit breaker can be opened without a substantial increase in the arcing time.

The voltage source converter may be configured as part of a bipolar power transmission system and/or as part of a High Voltage Direct Current (HVDC) power transmission system.

In another aspect, a method for fault handling of an electrical distribution network is provided, the method comprising:

during a normal operation mode, operating a voltage source converter to exchange electrical power with an AC system having a plurality of electrical phases via an interface device, the interface device comprising a transformer comprising a set of primary windings connected to the electrical phases of the AC system and a set of secondary windings connected to the voltage source converter;

wherein the interface device further comprises a fault module comprising an energy storage element connected in parallel with a resistive element between a reference voltage and a neutral point of one of the set of primary windings or the set of secondary windings;

the method further comprises the following steps: monitoring for faults; and upon detection of a fault, generating a command to open a respective AC circuit breaker for each of the phases of the first AC system.

The interface device may be implemented in any of the variations as discussed above. The fault may be a phase-to-ground fault in any one of: the set of secondary windings; a phase leg of the voltage source converter or a connection between the set of secondary windings and a phase leg of the voltage source converter. In case of such a fault, the method may further comprise generating a command to block the voltage source converter.

As noted above, the fault module may further comprise a surge arrester electrically connected in parallel with the energy storage element and the resistive element.

Drawings

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 illustrates one example of a power transmission network;

fig. 2 illustrates one example of a conventional connection of VSCs between an AC system and a DC system;

fig. 3 illustrates an example of a connection of a VSC between an AC system and a DC system including an interface device according to an embodiment;

fig. 4a and 4b illustrate plots (plot) of current and impedance through an AC circuit breaker during a fault for a conventional connection and via an interface device according to an embodiment; and

fig. 5a and 5b illustrate plots of breaker open time versus arc time for an AC circuit breaker.

Detailed Description

Fig. 1 shows an example of a power transmission network 100 using HVDC. The network 100 shown in fig. 1 comprises a DC system connected between two

AC systems

101 and 102; however, it should be understood that embodiments may be implemented in any type of AC/DC coupling or a wide variety of types of AC/DC couplings in a power transmission system.

The power network 100 comprises a first AC system 101. The first AC system 101 may comprise any electrical AC system and can be, for example, a power source such as a wind farm, hydroelectric power plant or other AC generator, or can be an AC network or grid for distribution/transmission of power. The first AC system 101 is connected to a first converter station 103, the first converter station 103 comprising at least one Voltage Source Converter (VSC) 104 for converting from AC to DC or vice versa. The first converter station 103 is connected to the second converter station 105 via DC transmission lines or poles (poles) 106-1 and 106-2. The second converter station 105 further comprises at least one VSC104 for converting between AC and DC or vice versa, and the second converter station 105 is coupled to a second AC system 102, which second AC system 102 can be a separate AC transmission/distribution grid or it can be a different part of the same AC network as the first AC system 101.

Fig. 1 illustrates a bipolar arrangement, wherein each of the first and

second converter stations

103, 105 has a pair of VSCs 104, one VSC104 being connected to one pole of the DC system, e.g. to the high side transmission line 106-1, and the other VSC104 being connected to a second pole of the DC system, e.g. to the low side transmission line 106-2 (however, sometimes two VSCs can be considered as the positive and negative halves of one bipolar VSC. Each VSC104 may be connected on the DC side to an associated DC pole and also to a defined DC voltage, such as ground. In such a bipolar system, each VSC104 is coupled to a

respective AC system

101 or 102, and thus there are two AC arms connecting the

converter station

103 or 105 to the

respective AC system

101 or 102. It will be understood that other arrangements are however possible. For example, in a symmetrical unipolar scheme, each

converter station

103 and 105 may include a single VSC104, the single VSC104 being connected on the DC side between the high side DC terminal and the low side DC terminal (e.g., between lines 106-1 and 106-2), the single AC arm being connected to a respective AC system. Embodiments may be implemented in any type of HVDC scheme.

In practice, an AC system comprises a plurality of electrical phases, and typically three phases, and thus each AC arm comprises a path for each of the phases.

The

converter station

103 or 105 is connected to the

AC system

101 or 102 via a transformer 107, the transformer 107 having a set of primary windings connected to the AC system and a set of secondary windings connected to respective phase branches of the converter device 104. An AC circuit breaker system 108 is connected in each AC arm, the AC circuit breaker system 108 being operable to interrupt the circuit and stop current flow in the event of a fault being detected under the control of a fault controller (not illustrated in fig. 1).

Fig. 2 illustrates in more detail the structure of the VSC104 of one of the

converter stations

103 or 105 and the conventional connection of the VSC104 to the

AC system

101 or 102, wherein similar components as discussed with reference to fig. 1 are identified with the same reference numerals.

Fig. 2 (on the left hand side) illustrates the connections to the AC system, and in particular shows the polyphase components of the AC system labeled A, B and C. Each phase is input to a respective

AC circuit breaker

108a, 108b, 108c (collectively 108), each of which receives and is controlled by a control signal from the fault controller 201. The fault controller 201 is illustrated as a separate controller forming part of the

converter station

103 or 105, but in some examples at least part of the functionality of the fault controller may be provided by other components within the broader power network 100.

Each of the AC phases A, B and C is then coupled to the transformer 107. The transformer 107 is operable to transform the voltages of the AC system to values required for operation of the VSC 104. The transformer includes a set of primary windings 202 coupled to the AC system via the circuit breaker 108 and a set of secondary windings 203 coupled to the phase branches of the VSC 104. In the illustrated embodiment, primary winding set 202 is in a star configuration with a neutral point N, while secondary winding set 203 is in a delta configuration. The neutral point N may be connected to ground directly or via a limiting element, such as an impedance or surge arrester. A "star-delta" configuration arranged in this manner can provide some advantages, such as preventing third harmonic currents from flowing into the power supply line. However, alternative configurations may be used in some implementations.

Each of the transformed AC phases is connected to a respective phase branch of the converter device 104. In the illustrated embodiment, the converter device 104 is a Voltage Source Converter (VSC), and in general, the VSC104 may be any type of VSC, such as a half-bridge modular multilevel converter (HB MMC) as illustrated.

The VSC104 comprises respective up-converter arms 204 and down-converter arms 205. The upper converter arm 204 for a phase leg is connected between the AC node for that phase and the high side DC bus 206, and the lower converter arm 205 is connected between the AC node and the low side DC bus 207. For a bipolar configuration, one of the high-side or low-side DC buses may be connected to a reference potential, such as ground, and the other DC bus may be connected to one of DC lines 106-1 or 106-2.

Each

converter arm

204 or 205 may comprise an inductor 208 connected in series and one or more switching cells 209 also arranged in series. In some VSC designs, such as Modular Multilevel Converters (MMC), the cell 209 may comprise a storage element such as a capacitor 210 and a switch such as an IGBT 211 as illustrated, which can be controlled to connect the capacitor 210 in series between the terminals of the cell or to connect the terminals in a path that bypasses the capacitor. The series connection of such cells, sometimes referred to as a chain-link circuit or chain-link converter, allows the cells to be controlled collectively to provide voltage waveform shaping across the converter arms, facilitating (allow for) efficient conversion between AC and DC with relatively low levels of harmonic distortion.

In use, a fault may be generated within the

converter station

103 or 105. One type of fault is a single-phase ground fault, in which one of the AC phases is shorted to ground. In case such a fault is detected, the fault controller 201 may generate an open command for the AC circuit breaker 108.

As noted above, the AC circuit breaker 108 operates by quickly separating two contacts that are in physical contact in a normally closed state. The high voltages and currents involved at the beginning of the separation of the contacts may mean that an electric field is generated which is sufficient to ionize the gas or other medium (e.g. air) surrounding the contacts and arcing may begin. Once arcing is initiated, arcing can be maintained as long as current is still supplied to the AC circuit breaker, and can only be extinguished by a zero crossing of the current.

A problem may arise if a phase ground fault occurs in the connection of a part of the VSC104 and a part of the VSC104 to the AC system illustrated by the dashed box 212 in figure 2. This part of the converter station comprises the set 203 of secondary windings of the transformer 107, the inductor 208 in each

arm

204, 205 of the converter 104 and all connections therebetween. In general, the highlighted area 212 includes all components between the secondary winding 203 and the cell 209 of each

converter arm

204 or 205.

If such a phase-to-ground fault occurs in this part of the

converter station

103 or 105, it may cause a considerable fault current to flow within this phase on the secondary side of the transformer 107 (i.e. on the VSC side of the transformer). This in turn may result in a substantial injection of DC current into one or both of the phases A, B, C on the primary side of the transformer 107. The DC current provides an offset to the AC current flowing in the phase. Thus, at normal zero crossings of the current for the AC waveform, there may still be a DC current component flowing. The actual zero crossing of the current in the AC phase on the primary side of the transformer 107, and thus through the corresponding AC circuit breaker 108 for that phase, may be delayed to a later stage in the AC cycle, or may not occur for several AC cycles, or in some cases, may not have a zero crossing at all. This poses a serious problem for the proper operation of the AC circuit breaker 108, as the circuit breaker relies on the zero crossing of the current to extinguish any arcing and thus successfully open to stop the current flow.

In general, one phase of an AC system may still experience a zero crossing of current in the event of a phase to ground fault. If the relevant phase can be identified, a command to open the AC circuit breaker 108a-c of the relevant phase can be generated. Once the AC circuit breaker has been successfully opened, the current across the phase is successfully extinguished, thereby isolating the AC phase from the VSC 104. This will result in a reduction of the current in the remaining phases. The drive voltage in the faulted phase decreases and therefore the DC component of the current in the remaining phase will start to decrease and possibly start to experience a zero crossing.

Thus, as soon as a fault is detected, the AC circuit breakers 108 can be commanded to open, and once the current has dropped enough for a zero crossing to occur, the successful opening of one AC circuit breaker can allow the other AC circuit breakers to open. However, there will be a delay between the opening time of the first AC circuit breaker and the fault current component dropping sufficiently that all remaining AC circuit breakers are successfully opened. This delay time can impose an increased strain (strain) on the AC circuit breaker 108 because the breaker experiences a longer arcing time during the time delay until the zero crossing occurs. To avoid excessive arcing time, the AC circuit breakers can be commanded to open in a staggered manner when it is determined that a zero crossing for that phase is likely to occur. This may mitigate the amount of arcing time for each AC circuit breaker, but requires a relatively complex control scheme. In any case, the delay allows more time for any fault current to interact with, for example, the VSC104 or various components connected to the VSC104, thus allowing a greater likelihood of damage to the components.

Embodiments of the present disclosure alleviate this problem by using an interface device that includes a fault module coupled to a set of primary windings of a transformer. The fault module is connected between the neutral point of the primary winding set and a reference potential, which may be ground potential.

Fig. 3 illustrates connections and VSCs to an AC system including an interface device according to an embodiment of the present disclosure, where components similar to those illustrated in fig. 1 and 2 are identified by the same reference numerals.

Fig. 3 again illustrates the VSC104 coupled to the three phases A, B and C of the AC system via the transformer 107. In this embodiment, however, transformer 107 has a fault module 301 connected thereto. The fault module 301 includes an energy storage element 302, the energy storage element 302 being electrically connected in parallel with a resistive element 303 between a first module node and a second module node. In the embodiment illustrated in fig. 3, the energy storage element comprises at least one capacitor 302. The first module node of the fault module 301 is connected to the neutral point of the primary winding set 202. The second module node is connected to a reference node at a reference potential, which in normal operation is the same potential as the neutral point of the primary winding (in this example, ground). In some embodiments, the fault module further comprises a surge arrester 304, the surge arrester 304 being electrically connected in parallel with the resistive element 303 and the energy storage element 302.

Thus, the fault module 301 in the embodiment of fig. 3 is connected to the primary side of the transformer 107. In some instances, however, the fault module can instead be connected to the neutral point of the secondary winding set of the transformer to provide the same general advantages as described herein. The fault module 301 includes purely passive components and does not require any components such as switches or the like that require active control.

During normal non-fault operation, with all phases A, B and C connected to the primary winding 202, the voltages from all phases will be properly balanced. Thus, the voltage at the neutral point N of the primary winding set 202 will be substantially the same as the reference potential, and thus the presence of a faulty module will not substantially affect normal operation.

However, in the event of a phase ground fault in the portion 212 of the VSC104, the transformer secondary winding 203 or the connections therebetween, the presence of the components of the fault module can reduce the time required to open all of the AC circuit breakers 108.

In case of such a fault, the fault controller 201 may issue an open command to all AC circuit breakers 108. As noted above, in such phase-to-ground faults, there may be a DC current component imposed on one or more of the AC phases. However, at least one AC phase will still experience zero crossings of current at the usual intervals for an AC waveform. Thus, one of the AC circuit breakers 108a-c will successfully open within a relatively short period of time, isolating that phase from the VSCs.

Opening one of the circuit breakers 108a-c effectively removing the contribution of one phase will affect the voltage at the neutral point N of the primary winding 202. The voltage at the neutral point is no longer balanced between all phases and a non-zero voltage will be generated across the energy storage element (e.g., capacitor 302) and the resistive component 302. This voltage induces a current flow to/from capacitor 302. This current also flows in the AC phase in which the AC circuit breaker has not yet opened. This provides a modulation of the current distribution in such phases so that the influence of the fault current component is at least partly cancelled out. As such, this reduces the offset of the current in the residual phase. Thus, once one of the AC circuit breakers has opened, the presence of the fault module 301 causes the other AC phases to experience a zero crossing of the current earlier than would otherwise result as described above. The resistive component 303 is connected in parallel with the energy storage element so as to allow a path for DC current to flow when necessary, for example to provide a path for any DC current to enter the transformer neutral point during normal operation. This means that the fault module does not interfere with the normal operation of the transformer, but during a fault condition the current distribution of the AC phase can be modified as described above. Surge arresters 304 are provided to protect the transformer neutral and also limit electrical stress on the capacitors and resistors.

To illustrate the advantages of embodiments of the present invention, the operation of a conventional VSC arrangement such as that illustrated in figure 2 is simulated and compared with that of the arrangement illustrated in figure 3.

Fig. 4a illustrates the current values and impedance values through the circuit breaker 108 for each of the phases A, B and C in response to a detected fault using conventional fault handling methods. The top plot of fig. 4a shows the AC current waveform and the bottom plot shows the impedance of the circuit breaker. As mentioned above, upon occurrence of a fault (such as a single-phase ground fault), one of the phases will still experience a zero-crossing of the current. In the example illustrated in fig. 4a, phase B continues to experience a zero crossing. If all three AC circuit breakers are commanded to open, circuit breaker 108b will open first. Closing AC breaker 108b reduces the current offset of the other phase and phase a experiences a zero crossing shortly thereafter. However, the current of phase C still has a substantial DC offset and is subject to several cycles before it experiences a zero crossing when the circuit breaker 108C is able to stop conduction. Thus, the time taken to completely extinguish the fault current is relatively long and a rather long arcing period of more than 40ms is experienced for the AC circuit breaker of phase C.

Fig. 4b illustrates current values and impedance values through the circuit breaker 108 for each of the phases A, B and C in response to a detected fault using embodiments of the present disclosure. Fig. 4b shows again in the top plot the AC current through each phase of the AC circuit breaker and in the bottom plot the impedance of the circuit breaker. As in the previous example, a fault, such as a single phase earth fault, is detected and all AC circuit breakers are commanded to open. Phase B again experiences a zero crossing of the current and thus the circuit breaker 108B is the first to be fully open. However, in this example, once circuit breaker 108b opens, the voltage imbalance at the neutral point induces current to/from transformer fault module 301, which reduces the effects of DC offset due to fault current, as described. In this example, phase a again experiences a zero crossing shortly after the circuit breaker for phase B opens. However, as can be clearly seen, in this example phase C experiences far fewer cycles before experiencing a zero crossing of the current. The earlier zero crossings extinguish any current flow across the associated circuit breaker much faster than conventional approaches, thus reducing the risk of fault currents causing damage to various components of the system.

Fig. 5a and 5b illustrate plots of breaker open time versus arc time during a fault for different open times of an AC circuit breaker at different points in an AC cycle. It will be appreciated that the time taken for an AC circuit breaker to actually open may vary. Thus, fig. 5a and 5b consider many different breaker open times (in this example fourteen at 1ms intervals), and for a fault occurring at a given point in the AC cycle, the resultant arcing time for the worst-case AC phase is determined. Thus, for a fault occurring at a given point in the AC cycle, each point represents the resultant arcing time for a given breaker open time. Fig. 5a shows a response from a circuit breaker using a conventional fault handling method for VSCs. It can be seen that the arcing time for various AC circuit breakers may vary significantly and the arcing time for at least one of the AC circuit breakers may be as high as around 85 ms. Fig. 5b illustrates breaker open time versus arc time for a system having a transformer fault module according to an embodiment of the present disclosure. In this case, the worst-case arcing time of the circuit breaker is less than 40ms, and in this example, no more than about 36 ms. This reduced arcing time extinguishes the current across the circuit breaker more quickly, which imposes less strain on the circuit breaker and reduces the risk of damage.

Thus, embodiments provide an apparatus and method for fault handling in a VSC system. The interface device includes a fault module having a passive component, an energy storage element, and a resistive element connected in parallel between a neutral point of the transformer and a reference (e.g., ground) on a primary side of the transformer. For example, for a VSC forming part of a symmetric unipolar system or a bipolar arrangement as illustrated in fig. 1, an embodiment with such a fault module may be implemented in any type of HVDC scheme. Embodiments of the present disclosure allow a fault controller to simply issue an open command to all AC circuit breakers in the event of a fault, and ensure earlier induced zero crossings of the current in the AC phase, with reduced arcing time across the circuit breakers. Thus, embodiments isolate the AC power network from the VSC experiencing the fault in a quick and efficient manner. This reduces the risk of any fault current damaging components of the HVDC system, and in particular reduces the strain potentially experienced across the circuit breaker.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. The word "comprising" does not exclude the presence of elements or steps other than those listed in a claim, "a" or "an" does not exclude a plurality, and a single processor or other unit may fulfil the functions of several units recited in the claims. Any reference signs in the claims should not be construed so as to limit their scope.

Claims

1. An interface device for interfacing between an AC system (101, 102) and a DC system (106-1, 106-2), the device comprising:

a transformer (107), the transformer (107) having a winding set (202) for coupling to a plurality of AC phases (A, B, C) of the AC system, the winding set having a neutral point (N); and

a fault module (301), the fault module (301) comprising an energy storage element (302) connected in parallel with a resistive element (303) between a first module node and a second module node;

2. The interface apparatus of claim 1, wherein the fault module comprises a surge arrester (304), the surge arrester (304) being electrically connected between the first module node and the second module node in parallel with the energy storage element and the resistive element.

3. Interface device according to claim 1 or 2, wherein the energy storage element comprises at least one capacitor (302).

4. The interface device of any preceding claim, further comprising a respective AC circuit breaker (108) for each of the AC phases of the AC system.

5. Interface device according to claim 4, comprising a fault controller (201), the fault controller (201) being adapted to operate the AC circuit breaker in case of a fault.

6. The interface device of claim 5, wherein the set of windings of the transformer is a set of primary windings of the transformer.

7. The interface device according to claim 6, further comprising a set of secondary windings (203) of the transformer (107) for coupling to a plurality of phase branches of a voltage source converter.

8. The interface device of claim 7, wherein the fault controller is configured to command the AC circuit breaker to open in the event of a phase ground fault in any one of: the set of secondary windings; a phase leg of the voltage source converter or a connection between the set of secondary windings and a phase leg of the voltage source converter.

9. An interface device according to claim 8 wherein in the event of such a fault, the fault controller is further configured to send a command to block the voltage source converter.

10. An interface device according to any preceding claim, wherein the voltage source converter is configured as part of a bipolar power transmission system (100).

11. An interface apparatus according to any preceding claim, wherein the voltage source converter is configured as part of a High Voltage Direct Current (HVDC) power transmission system (100).

12. A method for fault handling of an electrical distribution network, the method comprising:

during a normal operation mode, operating a voltage source converter (104) to exchange electrical power with an AC system (101, 102) having a plurality of electrical phases (A, B, C) via an interface device, the interface device comprising a transformer (107), the transformer (107) comprising a set of primary windings (202) connected to the electrical phases of the AC system and a set of secondary windings (203) connected to the voltage source converter;

wherein the interface device further comprises a fault module (301), the fault module (301) comprising an energy storage element (302), the energy storage element (302) being connected in parallel with a resistive element (303) between a reference voltage and a neutral point of one of the primary winding set or the secondary winding set;

the method further comprises the following steps: monitoring for faults; and upon detection of a fault, generating a command to open a respective AC circuit breaker (108) for each of the phases of the first AC system.

13. The method of claim 12, wherein the fault is a phase-to-ground fault in any one of: the set of secondary windings; a phase leg of the voltage source converter or a connection between the set of secondary windings and a phase leg of the voltage source converter.

14. A method according to claim 12 or 13, wherein in case of a fault the method further comprises generating a command to block the voltage source converter.

15. The method of any of claims 12 to 14, wherein the fault module further comprises a surge arrester (304) electrically connected in parallel with the energy storage element and the resistive element.